Concrete dates back at least to Roman times. The invention of concrete allowed the Romans to construct building designs, such as arches, vaults and domes that would not have been possible without the use of concrete. Roman concrete, or opus caementicium, was made from a hydraulic mortar and aggregate or pumice. The hydraulic mortar was made from either quicklime, gypsum or pozzolana. Quick lime, also known as burnt lime, is calcium oxide; gypsum is calcium sulfate dihydrate and pozzolana is a fine, sandy volcanic ash (with properties that were first discovered in Pozzuoli, Italy). The concrete made with volcanic ash as the pozzolanic agent was slow to set and gain strength. Most likely the concrete was build up in multiple layers on forms that had to stay in place for a very long time. Although the concrete was slow to set and gain strength, over long periods of time it achieved great strength and was extremely durable. There are still Roman concrete structures standing today as a testimony to the quality of the concrete produced over 2000 years ago.
Modern concrete is composed of one or more hydraulic cements, coarse aggregates, and fine aggregates. Optionally, modern concrete can include other cementitious materials, inert fillers, property modifying admixtures and coloring agents. The hydraulic cement is typically portland cement. Other cementitious materials include Fly Ash, slag cement and other known natural pozzolanic materials. The term “pozzolan” is defined in ACI 116R as, “ . . . a siliceous or siliceous and aluminous material, which in itself possesses little or no cementitious value but will, in finely divided form and in the presence of moisture, chemically react with calcium hydroxide at ordinary temperatures to form compounds possessing cementitious properties.”
Portland cement is the most common hydraulic cement used around the world today. Portland cement is typically made from limestone. Concrete or mortar made with portland cement sets relatively quickly and gains relatively high compressive strength in a relatively short time. Although significant improvements have been made to the process and efficiency of portland cement manufacturing, it is still a relatively expensive and highly polluting industrial process.
Fly ash is a by-product of the combustion of pulverized coal in electric power generation plants. When the pulverized coal is ignited in a combustion chamber, the carbon and volatile materials are burned off. When mixed with lime and water, Fly Ash forms a compound similar to portland cement. Two classifications of Fly Ash are produced according to the type of coal from which the Fly Ash is derived. Class F Fly Ash is normally produced from burning anthracite or bituminous coal that meets applicable requirements. This class of Fly Ash has pozzolanic properties and will have minimum amounts of silica dioxide, aluminum oxide and iron oxide of 70%. Class F Fly Ash is generally used in hydraulic cement at dosage rates of 15% to 30% by weight, with the balance being portland cement. Class C Fly Ash is normally produced from lignite or subbituminous coal that meets applicable requirements. This class of Fly Ash, in addition to pozzolanic properties, also has some cementitious properties. Class C Fly Ash is used in hydraulic cement at dosage rates of 15% to 40% by weight, with the balance being portland cement.
Recently, the U.S. concrete industry has used an average of 15 million tons of Fly Ash at an average portland cement replacement ratio of approximately 16% by weight. Since Fly Ash is a by-product from the electric power generating industry, the variable properties of Fly Ash have always been a major concern to the end users in the concrete industry. Traditionally, wet scrubbers and flue gas desulfurization (“FGD”) systems have been used to control power plant SO2 and SO3 emissions. The residue from such systems consists of a mixture of calcium sulfite, sulphate, and Fly Ash in water. In using sodium-based reagents to reduce harmful emissions from the flue gas, sodium sulfite and sulfate are formed. These solid reaction products are incorporated in a particle stream and collected with the Fly Ash in particulate control devices. There is the potential for the sodium-based reagent to react with other components of the gas phases and with ash particulates in the flue gas and in the particulate control device. All of the products of these reactions have the potential to impact the resulting Fly Ash. Anecdotal evidence has shown that the Fly Ash that contains sodium-based components has unpredictable and deleterious effect in concrete. Consequently, the concrete industry is at great risk of using a product that is unpredictable in its performance. Coupled with the closure of many coal-fired power plants, resulting in less availability of Fly Ash, the concrete industry is facing a dramatic shortage of a familiar pozzolan.
Known natural pozzolans can be used in concrete to replace the growing shortage of Fly Ash. However, known natural pozzolan deposits are limited and generally are far from construction markets. Natural pozzolans can be raw or processed. ASTM C-618 defined Class N natural pozzolans as, “Raw or calcined natural pozzolans that comply with the applicable requirements for the class as given herein, such as some diatomaceous earth; opaline chert and shales; tuffs and volcanic ashes or pumicites, any of which may or may not be processed by calcination; and various materials requiring calcination to induce satisfactory properties, such as some clays and shales.”
Other known natural pozzolans include Santorin earth, Pozzolana, Trachyte, Rhenish trass, Gaize, volcanic tuffs, pumicites, diatomaceous earth, and opaline shales, rice husk ash and Metakaolin. Santorin earth is produced from a natural deposit of volcanic ash of dacitic composition on the island of Thera in the Agean Sea, also known as Santorin, which was formed about 1600-1500 B.C. after a tremendous explosive volcanic eruption (Marinatos 1972). Pozzolana is produced from a deposit of pumice ash or tuff comprised of trachyte found near Naples and Segni in Italy. Pozzolana is a product of an explosive volcanic eruption in 79 A.D. at Mount Vesuvius, which engulfed Herculaneum, Pompeii, and other towns along the bay of Naples. The deposit near Pozzuoli is the source of the term “pozzolan” given to all materials having similar properties. Similar tuffs of lower silica content have been used for centuries and are found in the vicinity of Rome. In the United States, volcanic tuffs and pumicites, diatomaceous earth, and opaline shales are found principally in Oklahoma, Nevada, Arizona, and California. Rice husk ash (“RHA”) is produced from rice husks, which are the shells produced during the dehusking of rice. Rice husks are approximately 50% cellulose, 30% lignin, and 20% silica. Metakaolin (Al2O3:2SiO2) is a natural pozzolan produced by heating kaolin-containing clays to a temperature of about 600 to 900° C. (1100 to 1650° F.) above which it recrystallizes, rendering it mullite (Al6Si2O13) or spinel (MgAl2O4) and amorphous silica (Murat, Ambroise, and Pera 1985). The reactivity of Metakaolin is dependent upon the amount of kaolinite contained in the original clay material. The use of Metakaolin as a pozzolanic mineral admixture has been known for many years, but has grown rapidly since approximately 1985.
Natural pozzolans were investigated in this country by Bates, Phillips and Wig as early as 1908 (Bates, Phillips,and Wig 1912) and later by Price (1975), Meissner (1950), Mielenz, Witte, and Glantz (1950), Davis (1950), and others. They showed that concretes containing pozzolanic materials exhibited certain desirable properties such as lower cost, lower temperature rise, and improved workability. According to Price (1975), an example of the first large-scale use of portland-pozzolan cement, composed of equal parts of Portland cement and a rhyolitic pumicite, is the Los Angeles aqueduct in 1910-1912. Natural pozzolans by their very definition have high silica or alumina and silica content either in a raw or calcined form.
Generally Fly Ash has the advantage that it can reduce water demand of the cementitious matrix. This reduces plastic shrinkage and allows for better workability. Generally, known natural pozzolans and silica fume increase water demand in the cementitious matrix; in some cases as high as 110%-115% that of portland cement. Greater water demand creates undesirable concrete properties such as lower strength development and greater plastic shrinkage. It is desired that pozzolans have a water demand that is lower than or equal to portland cement. However this is an extremely rare occurrence for known natural pozzolans.
Due to the wide variety of natural pozzolanic types and quality found in generally relative small deposits and contamination with other minerals makes it is difficult to provide consistent natural pozzolan materials on an industrial scale for a price comparable to the Fly Ash with similar and guaranteed performance required by the concrete industry. In addition since most of these deposits are found in the western part of the U.S., the transportation cost makes them prohibitive to use in the rest of the country. Therefore it would be desirable to have sources of natural pozzolan distributed throughout the country. It would also be desirable to have natural pozzolan having generally stable reactivity based on consistent chemical properties.
Aggregate quarries that mine construction aggregate are ubiquitous throughout the country. These aggregates chemical composition is primarily based on silicon dioxide and have the chemical component to react in a similar fashion as pozzolans. However these rock deposits are generally of crystalline type and are very slow to react even when ground to a sufficiently small particle size similar to other pozzolans or Fly Ash. While they may pass various sections of the ASTM C 618, overall they fail to meet other criteria. For example, these aggregates of a particle size sufficient to pass particle size criteria of a maximum of 34% of the amount retained when wet-sieved on a 45-μm (No. 325) sieve; they may pass the minimum requirement of 70% for total sum of silicon dioxide, iron oxide and aluminum oxide (SiO2+Al2O3+Fe2O3), they may pass the requirement for the loss of ignition of a maximum of 10% and pass the requirement of water demand of a maximum of 115% and the autoclave expansion or contraction of a maximum of 0.8%. Yet, they typically fail the strength activity index based on the reactivity criteria of a minimum of 75% of control with portland cement, at 7 days, and the a minimum of 75% of control with portland cement, at 28 days. In addition, while some aggregates may pass all of the above ASTM D-618 criteria they are well below the reactivity of portland cement or Fly Ash and therefore are undesirable for use in the market place.
The crystalline aspect of these aggregates may be changed to amorphous through calcination. However, calcination adds cost to the product and makes such process relative expensive. It would be desirable to alter the crystalline aspect of the fine particle size aggregate-based material by adding an amorphous component so that the reactivity index increases to pass ASTM C-618 at 7 and 28 days. It would also be desirable to convert a low reactive material through a relatively inexpensive process to enhance its reactivity index performance so that it can meet or exceed the reactivity properties of Fly Ash or other known pozzolans.